fix: detect Turbopack builds and set TURBOPACK env accordingly

When building with Turbopack (default in Next.js 16), the standalone output
only includes *-turbo.runtime.prod.js files, not the webpack variants. The
previous behavior of unconditionally setting TURBOPACK=false caused runtime
errors because module.compiled.js would try to load non-existent runtime files.

This change:
- Detects Turbopack builds by checking if only turbo runtime files exist in
  the traced files
- Sets TURBOPACK=true for Turbopack builds so the correct runtime is loaded
- Keeps TURBOPACK=false for Webpack builds (existing behavior)

Fixes runtime error: Cannot find module 'next/dist/compiled/next-server/pages.runtime.prod.js'

Author: relsunkaev

packages/open-next/src/build/patch/patches/patchEnvVar.ts

@@ -1,5 +1,5 @@
-import { createPatchCode } from "../astCodePatcher.js";
-import type { CodePatcher } from "../codePatcher";
+import { createPatchCode, patchCode } from "../astCodePatcher.js";
+import type { CodePatcher, PatchCodeFn } from "../codePatcher.js";
 
 /**
  * Creates a rule to replace `process.env.${envVar}` by `value` in the condition of if statements
@@ -21,6 +21,54 @@ fix:
   '${value}'
 `;
 
+/**
+ * Detects if the build was done with Turbopack by checking for turbo runtime files
+ * in the traced files list.
+ *
+ * Turbopack builds include files like:
+ * - pages-turbo.runtime.prod.js
+ * - app-page-turbo.runtime.prod.js
+ *
+ * Webpack builds include files like:
+ * - pages.runtime.prod.js
+ * - app-page.runtime.prod.js
+ */
+function isTurbopackBuild(tracedFiles: string[]): boolean {
+  // Check for turbo runtime files (contain "-turbo" followed by . or - in the name)
+  // Examples: pages-turbo.runtime.prod.js, app-page-turbo-experimental.runtime.prod.js
+  const turboFiles = tracedFiles.filter((file) =>
+    /-turbo[.-].*\.runtime\.(prod|dev)\.js$/.test(file),
+  );
+  const hasTurboRuntime = turboFiles.length > 0;
+
+  // Check for non-turbo runtime files (end with .runtime.prod.js but don't have -turbo)
+  // Examples: pages.runtime.prod.js, app-page.runtime.prod.js
+  const nonTurboFiles = tracedFiles.filter(
+    (file) =>
+      /\.runtime\.(prod|dev)\.js$/.test(file) && !/-turbo[.-]/.test(file),
+  );
+  const hasNonTurboRuntime = nonTurboFiles.length > 0;
+
+  // If we have turbo runtimes but no non-turbo runtimes, it's a Turbopack build
+  // This handles the case where Turbopack only includes turbo runtime files
+  if (hasTurboRuntime && !hasNonTurboRuntime) {
+    return true;
+  }
+
+  return false;
+}
+
+/**
+ * Creates a patch function that sets TURBOPACK env var based on the build type.
+ * For Turbopack builds, it sets TURBOPACK to true; for Webpack builds, it sets it to false.
+ * This ensures the correct runtime files are loaded at runtime.
+ */
+const createTurbopackPatch: PatchCodeFn = async ({ code, tracedFiles }) => {
+  const isTurbo = isTurbopackBuild(tracedFiles);
+  const value = isTurbo ? "true" : "false";
+  return patchCode(code, envVarRuleCreator("TURBOPACK", value));
+};
+
 export const patchEnvVars: CodePatcher = {
   name: "patch-env-vars",
   patches: [
@@ -39,12 +87,14 @@ export const patchEnvVars: CodePatcher = {
       contentFilter: /process\.env\.NODE_ENV/,
       patchCode: createPatchCode(envVarRuleCreator("NODE_ENV", '"production"')),
     },
-    // This patch will set `TURBOPACK` env to false to avoid loading turbopack related deps at runtime
+    // This patch will set `TURBOPACK` env based on whether the build was done with Turbopack.
+    // For Turbopack builds, only *-turbo.runtime.prod.js files exist, so we need TURBOPACK=true.
+    // For Webpack builds, only non-turbo runtime files exist, so we need TURBOPACK=false.
     {
       versions: ">=15.0.0",
       pathFilter: /module\.compiled\.js$/,
       contentFilter: /process\.env\.TURBOPACK/,
-      patchCode: createPatchCode(envVarRuleCreator("TURBOPACK", "false")),
+      patchCode: createTurbopackPatch,
     },
   ],
 };